Ok, I can see it being that Zod chose to die and allowed himself to get in the situation where Supes was forced to kill him. Possibly that he knew that that would hurt Clark more than anything and because Zod knew that even if he did kill every human on earth he'd still be purposeless, so he forces Clark to take him out.

He allowed himself to be put in the headlock.

Cause honestly, Zod was trading Supes blow for blow and he was only exposed for a few minutes. It did not really look like Superman's 33 years of absorbing the Sun made any difference whatsoever. It's like putting a 230lb trained military man of 10-20+ years in elite forces against a similarly sized man who grew up on a farm and did random jobs across the country.